A continuously variable transmission (CVT) is a type of power transmission that is capable of achieving infinite variability between its highest and lowest possible speed ratios. Unlike conventionally geared transmissions that use one or more planetary gear sets and multiple rotating and braking clutches to establish a desired discrete gear state, a CVT instead uses a variable-diameter pulley system or variator to transition anywhere within a calibrated range of speed ratios. The variator typically includes two pulleys interconnected via a chain or belt. The belt rides in a gap defined by conical faces of mating halves of the pulleys. One of the pulleys is typically connected to an engine crankshaft and thus acts as a driving/primary pulley. The other pulley is connected to the output shaft of the CVT to act as a driven/secondary pulley. One or more gears or planetary gear sets could be used on the input and/or output side of the variator.
In order to vary the speed ratio, a shift force is applied via one or more actuators to the primary and/or secondary pulleys. The shift force effectively squeezes the pulley halves together to change the width of the gap between the conical faces. Variation in the gap size, which is also referred to as the pitch radius, causes the belt to ride higher or lower within the gap. This in turn changes the effective diameters of the pulleys and, as a result, changes the speed ratio.
A CVT is a relatively efficient transmission design largely due to the variable speed ratio capability and relative structural simplicity as described above. However, the performance of a CVT may remain less than optimal in certain respects. For instance, a CVT typically transitions within its allowable range of speed ratios without providing much in the way of the physical and audible “shift feedback” typically provided to a driver during shifting of a conventional automatic transmission. Likewise, some CVT speed ratio changes may take an extended amount of time to complete in comparison to an automatic transmission shift. Such factors can adversely affect the subjective responsiveness and drive quality of a CVT-equipped vehicle.